The ligand, bis-β-diketone with an azobenzene bridge (4,4′-(4,4,4-trifluoro-1,3-butanedione)azobenzene, H2L), was prepared for the synthesis of a series of dinuclear lanthanide complexes with the formula [Ln2L3(DMSO)4] (Ln = Eu3+, Gd3+, Tb3+, and DMSO = dimethyl sulfoxide). X-ray crystallographic analysis reveals that the three complexes are triple-stranded dinuclear structures formed by three bis-β-diketonate ligands with two lanthanide ions (Ln3+). The trans-to-cis photoisomerization rates of the azobenzene group of the three [Ln2L3(DMSO)4] complexes in ethanol and acetonitrile solutions are similar to those of the pure H2L ligand and other azobenzene-containing mononuclear lanthanide complexes, but the trans-to-cis quantum yields (Φt→c = 10–3) are 1 order of magnitude smaller. The first-order rate constant for the cis-to-trans thermal isomerization at 50 °C of the H2L ligand is similar to those of azobenzene derivatives, while those for the [Ln2L3(DMSO)4] complexes (kiso = 10–4 s–1) are higher than those of the mononuclear azobenzene-containing lanthanide complexes. Furthermore, as the lanthanide ionic radius becomes smaller from Eu3+ to Gd3+ to Tb3+, the thermal isomerization rate constant decreases and the half-life increases. All these results are proposed to arise from the rigidity at both ends of the azo group by coordination to the dinuclear lanthanide ions and the different isomerization mechanisms. These are the first examples of bis-β-diketonate dinuclear lanthanide complexes with an azobenzene bridge and help illustrate the mechanism of azobenzene isomerization.

Five lanthanide complexes constructed from a stilbene derivative, (E)-N′,N′-bis­(pyridin-2-ylmethyl)-4-styrylbenzoyl hydrazide (HL), and two β-diketonates (2-thenoyltrifluoroacetonate, tta), with or without a trifluoroacetate anion (CF3CO2 –), namely, [Ln­(tta)2(HL) (CF3CO2)] [LnC45H32F9N4O7S2, Ln = La (1), Nd (2), Eu (3), or Gd (4)] and [Yb­(tta)2(L)] (YbC43H31F6N4O5S2 (5), L = deprotonated HL), were synthesized and characterized. Crystals of these five complexes were obtained and analyzed by single-crystal X-ray diffraction. These complexes all belonged to the monoclinic P21/c space group. For La3+, Nd3+, Eu3+, and Gd3+, the central lanthanide ion was nine-coordinate with a monocapped twisted square antiprism polyhedron geometry. The central Yb3+ ion of complex 5 was eight-coordinate with a distorted double-capped triangular prism polyhedron geometry. Among the five complexes, trans-to-cis photoisomerization of the stilbene group in gadolinium complex 4 showed the largest quantum yield. Complexes 2, 3, and 4 showed dual luminescence and photoisomerization functions. The luminescence change of complex 3 was reversible upon the trans-to-cis photoisomerization process. The sensitization efficiencies of luminescent europium complex 3 in acetonitrile solutions and in the solid state were 49.9 and 42.6%, respectively. These medium sensitization efficiencies led to the observation of simultaneous photoisomerization and luminescence, which further confirmed our previous report that photoisomerization of the stilbene group within complexes was related to the lanthanide ion energy level and whether a ligand-to-metal center or ligand-to-ligand charge-transfer process was present. In complexes 1–5, in addition to the intramolecular absorption transition of the ligand itself (IL, πHL–πHL * and πtta–πtta*), the presence of a ligand-to-ligand charge-transfer transition between tta and HL (LLCT, πtta–πHL * or πHL–πtta *) indicated whether the triplet-state energy of HL was able to transfer to the excited energy level of the lanthanide ions, leading to different extents of HL photoisomerization. These results provide an important route for the design of new dual-function lanthanide-based optical switching materials.

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